The present invention relates to slurry fuels and, more particularly, methods to enhance the characteristics of hydrothermally prepared slurry fuels.
Of all the coal-based alternative fuels, coal-water fuels (CWFs) appear the most promising. In general, CWF technology was developed to make coal usage more practical and environmentally acceptable, particularly in the "clean" formulation. CWFs were developed as a direct replacement for oil, not as a replacement for dried coal. Even so, CWF has distinct advantages over pulverized coal in many applications; one basic advantage is that CWF is easier to handle, requiring less complicated equipment. This is especially true in pressurized systems such as advanced gasifiers, pressurized fluid-bed combustors, turbines, and diesel engines. Another advantage is that CWFs are nonhazardous, while pulverized coals produce dust and tend to combust spontaneously.
Commercial efforts to produce CWF from high-rank bituminous coals generally involve mixing finely ground coal and water, applying coal-specific cleaning procedures, followed by mechanical dewatering, if necessary, and final fuel formulation, at which time proprietary and most often costly additives are used to further enhance the product fuel. However, the processing steps for producing CWFs from low-rank coals (LRCs) are much different from those used for high-rank coals, since they must accommodate the high inherent moisture content of the LRCs. A hydrothermal treatment process, also known as hot-water drying (HWD), is one way to successfully produce high-grade CWF from LRCs. In the process, nature's coalification process is essentially accelerated. Exposing the coal to elevated temperatures and pressures for a time scale of minutes rather than geological eras produces irreversible changes, such as the evolution of CO.sub.2, release of bound cations, and tar sealing of micropores. These changes reduce the equilibrium moisture and hydrophilicity of the coal. Meanwhile, the inherent advantages of the LRCs, including the amount of volatiles and properties and structure of the char, are maintained, preserving their high reactivity and nonagglomerating tendencies. These advantages are very important when CWF is considered as a replacement for oil. However, even with hydrothermal treatment, most LRC slurries are only of marginal quality for energy-related applications. Further enhancements are required to generate the quality of CWF for significant replacement of oil. Potential markets for LRC water slurries include power stations, industrial furnaces, and institutional heating plants, especially those originally designed to burn coal but now modified to use oil. Thus, there is a need in the art for improved methods for preparing high-grade slurry fuel from LRC and other carbonaceous materials.
To consider a system which includes slurry processing or handling, one must be aware of the interaction between the solid and carrier medium. Changes in slurry viscosity and other flow properties, because of variations in solids content and temperature, can drastically alter the energy or equipment needed to further process or handle the slurry. Slurry viscosity and stability depend on the energy of interactions among slurry particles and wetting properties of the solid. Also, the solids particle size, shape, and the concentration impacts properties of the dispersion itself. One characteristic that is common for suspended solids is flocculation, which is governed by the balance between the forces of attraction and repulsion between the particles. The gelling of solids inhibits the flow behavior of the slurry and also detrimentally affects the static stability of the mixture. The stability of a dispersion with respect to flocculation depends on the relative magnitude of the potential energy of attraction and that of repulsion of the particles involved.
In the area of development of CWF, obtaining maximum solids loading and stability of the coal in water has led researchers to produce surface conditioning agents. To use coal as a quasi-liquid fuel, the coal is crushed and pulverized to approximately 70% less than 200-mesh particle size. The coal is then mixed with water to a given viscosity, prior to the addition of the surfactant or dispersant material. The additives adjust the pH of the medium, limit flocculation, or surface coat the coal particle as a means of slurry flow enhancement. These adjustments sufficiently improve the handling characteristics enough to maintain pumpable fluid while increasing the solids loading in the carrier fluid by 2 to 5 wt %. This technology has been widespread for the enhancement of bituminous coal and water mixtures. These additives, when tested with low-rank solid material such as lignite coals, were only minimally effective in lowering the viscosity. The LRC slurries differ from bituminous coal slurries in oxygen/carbon ratio, moisture level, and porosity. Each of these contributed to the poor product performance of LRC slurry fuel technology.
Hydrothermal treatment or pressure cooking the LRC slurry has been demonstrated to be an effective method of lowering the oxygen:carbon ratio and also reducing the inherent moisture content of the coal. However, during the process, hydroaromatic compounds may create increased particle flocculation and inhibit the flow characteristic. Mixing the slurry at low speed (e.g., shear rates less than 10,000 sec.sup.-1) produces a slurry which is fundamentally unstable, flocculating rapidly to form a volume-filling network throughout the continuous phase. The water is essentially immobilized by the network of chains, and the coal-water mixture behaves as an elastic solid under low stress. The term gel is used to describe such systems. Thus, there is a need in the art for an improved method of preparing slurry fuels from carbonaceous materials that does inhibit the flow characteristics of the slurry.
While there are a number of problems that are encountered when attempting to utilize biomass, agriculture wastes, or other solid wastes for energy production, the heterogeneity of the material is the source of many of the problems. One characteristic of hydrothermal treatment is to homogenize the material into a more chemically and physically consistent slurry fuel. The pumpable slurry has the advantages of being easily transported and injected into utilization systems. Since its moisture content is controlled to a constant level, the need for constant process and excess air adjustments when utilizing the fuel for power generation is avoided. The homogeneity of the fuel also promotes more consistent emissions during combustion, an important factor in the much regulated waste-to-energy industry. Although hydrothermal treatment helps to produce a homogeneous slurry fuel, it has only a limited effect. As such, there is still a need in the art for even more effective methods of homogenizing biomass and other solid waste for producing high-quality, homogeneous slurry fuels for energy applications.
It can therefore be seen that there is a real and continuing need for the development of improved methods for preparing high-grade slurry fuels from LRC and other carbonaceous materials.
The primary objective of the present invention is the provision of improved methods for preparing high-grade slurry fuels that are efficient in operation.
Another objective of the present invention is the provision of improved methods for preparing hydrothermally treated slurry fuels suitable for use in energy-related applications as replacements for oil.
Another objective of the present invention is the provision of improved methods for preparing hydrothermally treated slurry fuels from carbonaceous materials that do not inhibit the flow characteristics of the slurry.
Still another objective of the present invention is the provision of more effective methods of homogenizing biomass and other non-coal carbonaceous materials for producing high grade, homogeneous slurry fuels for energy-related applications.
These and other features, objects, and advantages will become apparent to those skilled in the art with reference to the accompanying specification.